PROJECT SUMMARY For several decades, this and other laboratories have extensively investigated the causes and immune pathogenesis of systemic lupus erythematosus (SLE), the prototypic systemic autoimmune disease. Despite considerable advances, particularly with regard to abnormalities in the adaptive immune system, several questions remained unanswered, including why autoantibodies in this disease are so often directed against nuclear antigens, and what is the initial trigger for these aberrant responses occurring even under sterile conditions? Emerging knowledge of a diverse array of mammalian sensors for nucleic acids, and the demonstration by us and others that these sensors are principal participants in lupus pathogenesis, have now provided new avenues of inquiry as to how this disease (and possibly many others) is initiated. We recently observed that congenic lupus-predisposed mice carrying an inactivating mutation of the proton/histidine transporter SLC15A4 (termed feeble) showed significantly reduced disease manifestations. The feeble mutation, discovered by others through ENU mutagenesis, has been shown to extinguish signaling by the endolysosomal TLR7 and TLR9, together with almost complete absence of production of type I interferons (IFN-I) and other proinflammatory cytokines by plasmacytoid dendritic cells (pDCs) without affecting the development of these cells. Because of the apparent potential to therapeutically intervene with the function of SLC15A4, this proposal will seek to determine how the feeble mutation reduces autoimmunity by defining the functional characteristics of this molecule, the effects of the feeble mutation in lupus-associated innate and adaptive pathogenic responses, and ultimately to utilize high-throughput screening systems to identify pharmacologic inhibitors of this and other molecules necessary for self-nucleic acid sensing. The insights gained from these studies are certain to provide significant data on the biology of endolysosomes, TLRs and SLC15A4, and to reveal novel targets for therapeutic interventions in SLE and other autoimmune diseases.